回到顶部

 var btn = document.getElementById("btn");

        //首屏的高度加上滚动条卷去的高度大于3500的时候就显示btn
        //获取首屏的高度
        var winH = document.documentElement.clientHeight || document.body.clientHeight;

        window.onscroll = function () {
            //获取滚动条卷去的高度
            var scrollT = document.documentElement.scrollTop || document.body.scrollTop;
            if ((winH + scrollT) > 3500) {
                btn.style.display = "block";
            } else {
                btn.style.display = "none";
            }
        }


        //点击按钮滚动条回到顶部
        btn.onclick = function () {
            // this缓存
            var _this = this;
            window.clearInterval(this.timer);
            this.timer = window.setInterval(function () {
                // 获取
                var scrollT = document.documentElement.scrollTop || document.body.scrollTop;
                //设置
                document.documentElement.scrollTop = document.body.scrollTop = scrollT - 100;

                if (scrollT <= 0) {
                    document.documentElement.scrollTop = document.body.scrollTop = scrollT = 0;
                    //清除定时器
                    //this  ->window
                    window.clearInterval(_this.timer);
                }

            }, 30);
        }